1. Field of the Invention
The present invention relates generally to a method for fabricating a semiconductor device and, more particularly, to a method for removing an ion-implanted organic resin material layer, such as a resist layer, used as a mask in an ion-implantation process for doping impurities.
2. Description of the Related Art
With improvements in high integration and packing density of a semiconductor integrated circuit (IC) in recent years, an effort has been made to achieve ultra-miniaturization of semiconductor elements and wiring structures as structural elements. As a result, elements in size of 1 .mu.m or less in the smallest region are now in practical use. Such ultra-miniaturization causes amounts of adhesives and contaminants, such as fine dust in the micron size range, to determine the manufacturing yield of ICs in the manufacturing process. Therefore, manufacturing processes require removal of adhesives, residual substances and contaminants as much as possible.
Generally, contamination is reduced by using an ion-implantation impurity doping process. Also, residual substances of such contamination are reduced in the subsequent process for removing the ion-implanted mask.
In the ion-implantation method, it is not required to heat the substrate to a high temperature while the mask is attached, as opposed to other impurity doping methods (for example, the thermal impurity diffusion method). Therefore, it is often the case that an organic photoresist or a polyimide is used as the mask.
After ion-implantation, the mask of unwanted organic resin material layer is removed. However, it is known that the ion-implanted organic resin material layer partly changes its properties due, for example, to carbonization, and cannot be easily stripped. (See Kelvin J. Orvek and Craig Huffman; "CARBONIZED LAYER FORMATION IN ION IMPLANTED PHOTORESIST MASKS", Nuclear Instruments and Methods in Physics Research B7/8 (1985) 501-506)).
In the so-called wet method using a resist stripping solution, the carbonized organic resin material cannot be stripped. Therefore, an oxygen plasma ashing method is generally employed for removal of the ion-implanted organic resin material mask layer. In this case, not only the organic resin material but also carbon formed by carbonization of such material can be oxidized and removed by irradiation of an oxygen plasma.
The carbonized organic resin material layer is effectively removed by such oxygen plasma ashing method. However, when boron (B) or arsenic (As) ions are implanted in high concentrations with dose amounts of about 1.times.10.sup.14 /cm, such method still results in a problem in that a large amount of the organic resin material mask layer remains as a residual substance or stripping residue. If argon (Ar) is ion-implanted, comparatively less stripping residue is left. This is because, for one reason, the impurity itself, such as B or As ions implanted in the mask, is oxidized at the time of oxygen plasma ashing and becomes a non-volatile oxide.
Thus, a method has been developed which uses a plasma which contains hydrogen instead of oxygen, for example, a nitrogen-hydrogen mixing gas plasma, in order to remove B and Ar as volatile hydrogen compounds. According to this method, the stripping residue of the organic resin material mask layer can be reduced by a degree similar to that in the case of an Ar ion-implantation, even when B or As is ion-implanted in high concentrations. However, this method still results in a stripping residue of contaminants having a size of 1 .mu.m or more which is left in amounts as much as about 10.sup.4 per 1 cm.sup.2.
The reasons for causing such stripping residue in amounts noted above, and the methods for reducing the same have not heretofore apparent.